1. Field of the Invention
The present invention relates to a reflection type liquid crystal display device for displaying by reflecting the incident light.
2. Description of the Related Art
Recently applications of liquid crystal display devices in word processor, laptop personal computer, pocket-size television and others are rapidly advancing. In particular, among the liquid crystal display devices, the reflection type liquid crystal display device for displaying by reflecting the entering light is highly noticed because the power consumption is low since the backlight is not needed, and the design is thin and can be reduced in weight.
Hitherto, for the reflection type liquid crystal display device, the TN (twisted nematic) system and STN (super-twisted nematic) system have been employed, but in these methods, 1/2 of the luminous intensity of natural light is not utilized in display because of the use of the polarizer, and the display is dark.
To solve this problem, display modes for effectively utilizing all of rays of natural light without using polarizer have been proposed. An example of such modes is a phase transition type guest-host system (D. L. White and G. N. Taylor: J. Appl. Phys. 45 4718, 1974). In this mode, the cholesteric-nematic phase transition phenomenon due to electric field is utilized. By combining this system with micro color filter, a reflection type multicolor display is also proposed (Tohru Koizumi and Tatsuo Uchida, Proceedings of the SID, Vol. 29/2, 1988).
To obtain a brighter display in the mode not requiring polarizer, it is necessary to increase the intensity of light scattering in a direction vertical to the display screen, for the incident light from all angles. For this purpose it is needed to make a reflector having an optimum reflective characteristic. The above publication disclose a reflector manufactured by toughening the surface of substrate such as glass with abrasive, controlling the surface asperities by varying the time of etching with hydrofluoric acid, and forming a silver foil on the asperities.
FIG. 10 is a plan view of a substrate 2 possessing a thin film transistor (TFT) which is a switching element used in active matrix system, and FIG. 11 is a sectional view of XI--XI in FIG. 10. On an insulating substrate 2 of glass or the like, plural gate bus wirings 3 made of chromium, tantalum or the like are disposed parallel mutually, and gate electrodes 4 are branched off from the gate bus wirings 3. The gate bus wirings 3 function as scanning lines.
Covering the gate electrodes 4, a gate insulating film 5 made of silicon nitride (SiN.sub.x), silicon oxide (SiO.sub.x) or the like is formed on the entire surface of the substrate 2. On the gate insulating film 5 above the gate electrodes 4, a semiconductor layer 6 composed of amorphous silicon (a-Si), polycrystalline silicon, CdSe or the like is formed. At one end of the semiconductor layer 6, a source electrode 7 made of titanium, molybdenum, aluminum or the like is superposed. At the other end of the semiconductor layer 6, same as the source electrode 7, a drain electrode 8 made of titanium, molybdenum, aluminum or the like is superposed. At the opposite end of the drain electrode 8 against the semiconductor layer 6, a picture element electrode 9 made of ITO (indium tin oxide) is superposed.
As shown in FIG. 10, source bus wirings 10 crossing the gate bus wirings 3 across the gate insulating film 5 are connected to the source electrode 7. The source bus wirings 10 function as signal lines. The source bus wirings 10 are also made of the same metal as the source electrode 7. The gate electrode 4, gate insulating film 5, semiconductor layer 6, source electrode 7, and drain electrode 8 compose a TFT 1, and this TFT 1 possesses the function of switching element.
To apply the substrate 2 possessing the TFT 1 shown in FIG. 10 and FIG. 11 in a reflection type liquid crystal display device, it is necessary to form the picture element electrode 9 by using a metal possessing light reflectivity such as aluminum and silver, and form the gate insulating film 5 or asperities thereon. Generally, it is difficult to form tapered asperities uniformly on an insulating film made of inorganic matter.
As shown in FIG. 10 and FIG. 11, when forming the reflection electrode 9 and source bus wirings 10 on the gate insulating layer 5, a gap 9a is formed so as to prevent conduction between the reflection electrode 9 and source bus wirings 10. Or, when the reflection electrode 9 is formed on the TFT 1, the source electrode 7 and drain electrode 8 conduct, and the TFT 1 cannot function as switching element, so that the reflection electrode 9 cannot be formed on the TFT 1.
To enhance the luminance of display, it is desired that the reflection electrode 9 be as large as possible. However, as mentioned above, it must be formed so that the reflection electrode 9 may not contact with the source bus wirings 10, and many not overlap with others than the drain electrode 9 of the TFT 1, which results in, to the contrary, smaller surface area of the reflection electrode 9, hence lower luminaries and lower display quality of the reflection type liquid crystal display device.